Method of electroplating zinc on iron or steel



30 cle zinc-receptive,

Patented Mar; 3, 1931 UNITED STATES PATENT OFFICE SIDNEY H. DAVIS AND CARD 0. ANDERSON, F BAXTER SPRINGS, KANSAS; WILLIAM N. SMITH, OF PLATTEVILLE, WISCONSIN; AND HERBERT R. HANLEY, OF ROLLA, MISSOURI, ASSIGNORS TO THE CENTURY ZINC COMPANY, A CORPORATION OF OHIO METHOD OFELECTROPLATING ZINC ON IRON OR STEEL No Drawing. Application filed November 2, 1928, Serial No. 316,874. Renewed. August 21, 1930.

Our invention relates to the plating of ferrous'articles with zinc and more especially to the plating of such articles in relatively high acid solutions of zinc sulfate by means of currents of high densities delivered from anodes insoluble in such solutions, for example, in accordance with the method disclosed and claimed in an application for Letters Patent of the United States filed by us September 12, 1928 under Serial No. 305,429. As stated in that application, it has hitherto been customary to render the ferrous article zincreceptive preparatory to the actual plating operation by immersing it in an alkaline 1 cleaning solution of caustic soda or the like to effect removal of grease, to then rinse the article and then immerse it in a suitable pickling solution, ordinarily a dilute solution of sulphuric acid, to remove the scale and to then rinse and brush the article, with the result that a clean, bright surface is produced thereon to receive the zinc.

We have found, however, that the plating operation can be greatly facilitated and, what is of even more importance, the character of the plating and its resistance to corrosion in the presence of weak acids materially increased, if, during and as a part of the operations incident to rendering the ferrous artian extremely thin coating of lead in one form or another he formed upon the article in such manner that what may be for convenience termed a leady-ferrous surface, as distinguished from a clean steel oriron surface, be formed for the reception of the zinc. The surface to which we have reference, and as will'hereinafter more fully appear, is of a character distinct from a continuous, permanent plating or coating of lead entirely covering the ferrous base and which would thus in itself form a base for the reception of the zincwhen subsequently applied, substantially as the coating of copper which is frequently applied to does not of itself form a base and thus a distinct interposed layer or stratum between the brass or other non-ferrous articles prior to ferrous article and the plating of zinc but rather acomponent part of the ferrous base itself, intimately associated with the molecular structure thereof and thus forming therewith a compound leady-ferrous surface for the reception of the zinc.

Considered in its broader aspects therefore, our invention contemplates the provision of a novel method of electroplating ferrous articles with zinc by means of high acid solutions of zinc sulfate and high density currents delivered from anodes insoluble in such solutions, and, considered more specifically, in the provision in such a method of certain steps resulting in the creation on the article of a compound lead-ferrous surface preparatory to the series of steps or operations by which the deposit of the zinc is effected, with the object of facilitating the latter as well as of improving the quality and corrosion-resistant properties of the ultimate zinc plat- 1ng.

Other objects, advantages, novel features and characteristics comprehended by our invention are hereinafter more specifically mentioned or will be apparent to those skilled in the art as a more detailed understanding of the invention is acquired from the following description.

In accordance with our invention the incipient thin coating. of lead is desirably formed on the ferrous articles. after the latter have been-freed from foreign substances such as grease and scale in any suitable way and the formation and subsequent treatment of the coating may thus in one sense be considered as incident to and a part of the series of steps or operations designed to render the article zinc-receptive. Moreover, the said incipient coating of lead may beformed in various ways as will hereafter more fully appear, but irrespective of the particular method employed to form it, the coating is subsequently subjected to a vigorous brushing or bufling, desirably with a steel wire brush or like implement, so as to force a portion of the coating into the pores of the surface of the article and remove the excess of the coating and thereby properly condition the article for the reception of the zinc which may be app1ied thereafter by electrolysis of high acid solutions of zinc sulfate by means of currents of high density delivered from anodes insoluble in such solutions and preferably in accordance with the methods disclosed and claimed in our said application for U. S. Letters Patent Serial N 0. 305,429.

Reference will now be made to three different ways of performing our invention, the first of which however is ordinarily preferred: I

Example 1.The ferrous article to be plated, for example a steel sheet, is first immersed in an alkaline solution. such as a weak solution of caustic soda, to effect removal of any grease which may be present on the article and is then withdrawn from the solution and thoroughly washed. The article is then pickled for a suitable length of time in .an acid solution, such as a relatively dilute solution of sulphuric acid, to loosen and to some extent remove the scale from the surface of thearticle and is then withdrawn from the solution and again, washed, during which operation any scale which has been loosened but not entirely dissolved in the pickling bath is removed; brushing with a wire brush or the like may be resorted to if found necessary to entirely free the article from the scale. These cleaning and pickling operationsare well understood and thus require no further description.

The article is next immersed in an alkaline bath preferably containing caustic soda and sodium carbonate, about 15 grams of the former and 60 of the latter being desirably employed for each liter of solution, and a direct current then passed through this bath to the article acting as a cathode from an anode formed of some material insoluble or substantially so in the bath under the conditions intherefore be advantageously employed as dicated'; an anode or anodes of iron may they are cheap, readily manufactured and practically insolublein the bath in the presence of the electric current which is preferably of relatively low density as, for example, 30 amperes per sq. ft. of cathode surface, though currents of other density may be utilized if desired.

In addition to the alkaline ingredients a suitable quantity of lead either in the metallic state or as lead oxide, lead carbonate or other suitable lead compound is introduced into the bath to rovide the lead for forming the coating on t e ferrous article. For this purpose we prefer to utilize scrap metallic lead as the same is readily obtainable and can be readily introduced into the bath in the form of small blocks or pieces; a certain amount of this metallic lead dissolves in the bath and forms sodium plumbate or some generally similar compound. As the quantity of lead required in the solution is relatively small, under certain circumstances it may be deemed preferable to utilize lead tanks for the bath and thereby obtain all or a portion of the requisite amount of lead through the action of the solution on the tank itself. However, as under these conditions the tank itself is gradually eaten away and thus ultimately rendered useless, it is ordinarily preferable to use iron or other tanks unaffected by the alkaline solution and to supply the lead from time to time as may be required either as metallic lead or in some other form at least slightly soluble in the solution under the conditions present. While as stated, but a relatively small amount of lead is required in the solution, under ordinary conditions the greater the quantity thereof the less will be the time required to form the requisite coating on the article with a current of iven density; thus the quantity of leadsupplied to the bath will be generally determined by the particular operating conditions encountered but in any case it is desirable that enough lead in metallic or other-form be always present in the tank to insure a suflicient amount thereof in the solution or'bath to enable the production of therequisite coating within a reasonable time with the current which is being employed, such as in from two to ten minutes when using a current of 30 amperes density per sq. ft. of cathode surface.

The lead coating produced upon the ferrous article under the conditions described is black or substantially black in color and of a distinctly spongy texture and thus presents distinctly different physical characteristics from metallic lead.

The electrolysis of the bath having been continued until the surface of the ferrous article is thoroughly coated with the spongy lead deposit, the article is then removed from the bath and washed, and the coated surface pose and which may therefore be deemed as excess. There is thus formed upon the article a compound lead-ferrous surface for the reception of the zinc which may now be immediately applied to the article or the latter may be immersedin water to prevent oxidation until it be desired to apply the zinc. As hitherto stated the application of this metal in accordance with our invention is effected by electrolysis by currents of high density of high acid solutions of zinc sulfate utilizing the article as a cathode and an anode insoluble in such solutions, for example, anodes of lead, the method employed being preferably that disclosed in our said application Serial No. 305,429 and which briefly comprises the steps of forming a thinstrike coating of zinc upon the article by electrolysis of an acid solution of zinc sulfate by a current of high density delivered from an insoluble anode to the article acting as a cathode, then removing the article from that bath and continuing the plating thereof with zinc by electrolysis of another bath comprising a solution of zinc sulfate of higher acidity than the first bath by a current of relatively lower density than that previously employed, said current being delivered from anodes insoluble in the bath to the article acting as a cathode, until a plating of the desired thickness is attained to thereby produce a finished article of commerce.

E mample 2.:Instead of forming the spongy lead coating upon the ferrous article by electrolysis of an alkaline lead-containing solution as in Example 1, the coating may be formed, after freeing the article from grease and removing the scale by pickling and brushing as described in that example, by simply immersing the article in a solution of lead acetate or of .lead chloride for a suitable period. Under these conditions the formation of the coating is slow when the bath is cold but becomes more rapid as the temperature of the bath is raised, and the spongy lead coating formed in this manner exhibits substantially the same characteristics as to color and texture as that formed in the manner heretofore described, but since its formation even in a warm solution is slow and a considerable time is thus required to produce a coating of the requisite thickness, the electrolytic method of forming the coating to which we have hitherto referred will ordinarily be pre ferred.

After a coating of the requisite thickness has been obtained, the article is removed from the bath and subjected to the same treatment including washing, brushing and plating with zinc, as described in Example 1.

Example 3.The incipient coating of lead upon the ferrous article may also be formed after the article has been freed from grease and scale as described in Example 1 by electrolysis of a bath of a solution of lead fluosilicate containing some free fluosilicic acid, a solutionsuch as is customarily employed in electrolytic refining of lead being suitable for the purpose. The ferrous article having been freed from grease and scale is either immediately transferred to the lead fluosilicate soiution or kept in water so as to prevent oxidation during any appreciable interval before its transfer thereto. While in the lead fluosilicate bath the article acts as a cathode "and electrolysis of the solution is effected by passing to the article, preferably from a lead anode, a direct current of low density for a very short time as, for example, a currentof 12 amperes per sq. ft. of article surface for a period of from 15 to 45 seconds, This results in the formation on the article of a very thin coating of metallic lead which therefore differs in characteristics from the coatings produced as heretofore described, it being coherent as distinguished from spongy.

After the formation of this coherentcoating the article is removed from the bath, thoroughly washed and then vigorously brushed with a steel wire brush or like implement so as to remove the major or excess part of the lead coatin and cause a considerable portion of the balance to penetrate into the pores of the surface of the article, thus, as in the other examples, producing a lead-ferrous surface for the subsequent reception of the zinc which may then be plated thereon as in the foregoing examples.

-In accordance with the usual commercial practice in vogue today ferrous articles are coated with zinc by either of two methods, namely, by the hot dipping method or by electroplating in a neutral or substantially neutral solution of zinc sulfate or in an alkaline solution of cyanide of zinc by means of a current of relatively low density delivered from a soluble zinc anode to the article acting as a cathode, and the zinc plated ferrous articles which we are able to produce in accordance with our invention exhibit enormously greater resistance to corrosion inthe presence of weak acids than similar ferrous articles plated with zinc by either of these methods or by any other method with which we are familiar.

In'fact, numerous and comprehensive tests have shown that zinc plated ferrous articles produced in accordance with our invention display substantially one hundred fifty times greater resistance to corrosion than similar ferrous articles coated with the same amount of zinc by either of the two methods to which we have just referred, or, in other words, zinc plated ferrous articles produced in accordance with our invention will, under like conditions and with equal thicknesses of zinc plating, retain the latter in the presence of weak acids some one hundred fifty times longer than similar ferrous articles plated with a like amount of zinc by electroplating or hot dipping in the usual way. This result we attribute, at least in part," to the fact that the compound leady-ferrous surface which is formed for the reception of the zinc in accordance with our method exhibits an increased hydrogen over voltage with respect to the zinc than does a plain, clean ferrous surface with the result that the zinc deposits more readily thereon and adheres thereto with great tenacity. But whatever he the full explanation for the results we are able to attain, the fact remains that by the practice of our invention we are able to economically and conveniently produce zinc coated ferrous articles capable of retainin their zinc coating in the presence of wea acids for a much longer. time than those produced by any other method or process known to us.

It should be noted that irrespective. of the particular manner in which the incipient lead coating may be initially formed upon the ferrous article the said coating is so subsequently mechanically treated, as by brushing with a steel brush, that a very large portion thereof is entirely removed and at least a considerable portion of the balance forced into the pores of the surface of the ferrous base so that a compound surface of lead and steel or iron, more or less pervious and interrupted, is presented for the reception of the zinc in contradistinction to a continuous, impervious unbroken surface of either metal which would therefore in itself form a base for the reception of any subsequent coating or plating. Thus, an article produced in accordance with our invention may be considered as exhibiting three zones or strata, an inner stratum of ferrous material formed by the base itself, then a thin stratum in which ferrous material, lead and zinc are very intimately mixed, united and locked together, and then a third or outer, stratum of substantially pure metallic zinc. We are aware it has been proposed to plate ferrous articles with zinc by electrolysis of cyanide solutions containing mercury with a View to increasing the hydrogen overvoltage of the article and facilitating the deposition of the zinc thereon; under such conditions, however, the finished article ultimately produced exhibits a ferrous base having a coating or plating of a mercuric-zinc alloy of.substantially uniform composition throughout; our invention, on the other hand, neither contemplates, nor results in the production of, a mercuric or other alloy-zinc plating, but on the contrary and as distinguished therefrom, results in the production of a stratum or layer of substantially pure zinc upon a previously prepared surface of such character as to materially facilitate the deposit of the zinc thereon yet without interfering with an intimate bonding of the latter with the ferrous base. Having thus described our invention, we claim and desire to protect by Letters Patent of the United States: 1. The method of electroplating zinc on iron or steel which comprises the steps 'of forming on the surface of the article an inc1p1ent coating of lead, causing by mechanical means a portion of the coating to penetrate the surface of the article and removing the excess of the coating and then plating the article with zinc by electrolysis of an acid solution of zinc sulfate.

2. The method of plating zinc on iron or steel which comprises the steps of cleaning the surface of the article to be plated to remove foreign substances and scale, forming on said surface an incipient coating of lead, causing by mechanical means a portion of said coating to penetrate the surface of the article and removing the excess of the coating and then plating said surface with zinc by electrolysis of a high acid solution of zinc sulfate by a direct current of high density delivered from an anode insoluble in said solution.

3. The method of plating zinc on iron or steel which comprises the steps of cleaning the article from foreign substances and scale, forming a thin coating of lead on the surface of the article, removing a portion of the coating and causing another portion thereof to penetrate the surface'of the article by a brushing operation, and then plating said surface with zinc by electrolysis of a high acid solution of zinc sulfate.

4. The method of plating zinc on iron or steel which comprises the steps of cleaning the surface of the article to be plated to remove foreign substances and scale, forming a thin coating of lead on said surface, removing a portion of said coating and causing another portion thereof to penetrate said surface by a brushing operation, and then plating said surface with zinc, first, by electrolysis of an acid solution of zinc sulfate by a direct current of high density and then by electrolysis ofanother solution of zinc sulfate of higher acidity than the first solution by a direct current of lower density than the first current until a zinc plating of the desired thickness is obtained.

5. In a method of plating ferrous articles with zinc by electrolysis of a high acid solution of zinc sulfate, the steps of forming on the surface to be plated a thin coating of lead and then causing a portion of the coating to penetrate said surface to thereby form a compound lead-ferrous surface for the reception of the zinc.

6. In a method of plating ferrous articles with zinc by electrolysis of a high acid solutionof zinc sulfate, the steps of pickling the article in a dilute sulphuric acid solution to remove scale, then forming on the surface of the article a thin coating of lead, and finally by mechanical means removing a portion of said coating and causing another portion thereof to penetrate the surface of the article preparatory to plating the zinc thereon.

7. In ,a method of plating ferrous articles with zinc by electrolysis of a high acid solution of zinc sulfate, the steps of forming on the article'a thin coating of spongy lead by electrolysis of a lead-bearing alkaline solution and then removing a portion of said coating and causing another portion thereof to penetrate the surface of the article preparatory to plating the zinc thereon.

8. In a method of plating ferrous articles with zinc by electrolysis of a high acid solution of zinc sulfate, the steps of forming a thin coating of spongy lead on the surface to be plated by electrolysis of an alkaline leadbearing solution by current of low density delivered from an anode insoluble in said solution, and then brushing the article to remove a portion of said coating and cause another portion thereof to penetrate the surface preparatory to plating the zinc thereon. 

